Ballistic coefficient explained: G1, G7 and what BC means
Ballistic coefficient is a single number for how well a bullet resists air drag. This explains what it measures, why the same bullet has different G1 and G7 numbers, and how much a wrong BC shifts your dope at distance.
What BC describes
Ballistic coefficient (BC) is a measure of how efficiently a bullet flies — how slowly it sheds velocity to aerodynamic drag. A higher BC bullet holds its speed better, so over a given distance it drops less, drifts less in the wind, arrives sooner and carries more energy.
BC rolls together the things that make a bullet slippery: a long, sleek ogive, a boat-tail base, a small meplat (tip), and enough weight for its diameter (high sectional density). A 140-grain 6.5 mm match bullet might have a G1 BC around 0.6; a blunt 40-grain .22 LR bullet is nearer 0.12.
Why G1 and G7 numbers differ
A BC only means something relative to a standard projectile shape — a reference bullet whose drag curve is known. The published number tells you how your bullet compares to that reference.
- G1 is a short, flat-based reference shape. It is the historical default and still quoted for most hunting and rimfire bullets, but real modern bullets don't match its drag curve well, so a G1 BC drifts with velocity and is often given as a series of banded values.
- G7 is a long boat-tail reference shape that closely matches modern long-range match bullets. A single G7 BC stays accurate across the whole supersonic range, which is why serious long-range shooters prefer it.
The same bullet has a lower G7 number than G1 number — roughly half — because it is being compared to a more efficient reference. They are not interchangeable: always enter the BC and its matching drag model together, exactly as the manufacturer publishes them. In the calculator the drag-model selector sits right next to the BC field for this reason.
How much does a wrong BC matter?
Near your zero, almost not at all. The error grows with distance and accelerates as the bullet slows. As a rough guide, a 5 % BC error — easily the gap between a catalogue number and reality — is worth only a click or two at 300 yards but can be 0.3–0.5 MIL past 800 yards, i.e. a clean miss on a small target.
That is why truing exists: you shoot known distances, then adjust BC (or muzzle velocity) until the predicted trajectory matches your real impacts. A trued BC is specific to your rifle, barrel and lot.
BC vs muzzle velocity errors
Both push your dope off, but differently. A muzzle velocity error shifts the whole curve and shows up from mid-range outward. A BC error compounds with time of flight, so it stays hidden until long range and then dominates, especially near the transonic zone. If your near dope is good but your far dope is consistently off, suspect BC.